This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-268959 filed on Sep. 5, 2000.
1. Field of the Invention
The present invention relates to a brush holding device arranged in a direct current motor.
2. Description of Related Art
A direct current motor includes a commutator and brushes to mechanically commutate direct current supplied from an external direct current source and to supply it to the armature. The brushes are held in a brush holding device in such a manner that the brushes slidably engage against the commutator.
A previously proposed exemplary brush holding device 50 is shown in FIGS. 13 to 15. The brush holding device 50 includes a circuit board 51 and brush holders 52 (only one is shown). The circuit board 51 is secured to a motor housing (not shown). Each brush holder 52 is made of a heat-resistant metal material, such as a brass material, that exhibits thermally induced small dimensional change. Furthermore, each brush holder 52 is a hollow casing with a generally rectangular cross-section. A radially inner side (located on commutator 53 side) of the brush holder 52 is open, and a radially outer side (located on side opposite the commutator 53) of the brush holder 52 is closed.
Each brush holder 52 is secured to the circuit board 51 at its base portion 52a. A receiving recess 52b is formed in the brush holder 52 and has the open end on the commutator 53 side. A brush 54 and a spring 55 for urging the brush 54 against the commutator 53 are received within the receiving recess 52b of each brush holder 52.
When the commutator 53 rotates, each brush 54 exhibits self-excited vibrations. The self excited vibrations of the brush 54 is induced by fluctuating frictional force between the brush 54 and the commutator 53 and also by collisions between the brush 54 and a respective step formed between each two adjacent segments of the commutator 53. Because of the self-excited vibrations of the brush 54, the brush 54 repeatedly hits the corresponding brush holder 52, causing resonance of the brush holder 52 and generation of noises.
A first possible solution to the above-described disadvantage is to make each brush 54 from a relatively soft material. In this way, a collision force between the brush 54 and the respective step formed between each two adjacent segments of the commutator 53 is reduced, so that the self-excited vibrations of the brush 54 are accordingly reduced.
A second possible solution to the above-described disadvantage is disclosed in Japanese Unexamined Patent Publication No. 10-304625. According to this publication, a notch is formed in a lateral surface of each brush, and a vibration-isolating plate connected to a pig-tail is inserted within and is secured to the notch of the brush. Furthermore, a vibration absorbing material is placed between the vibration-isolating plate and a circuit board. With this arrangement, the self-excited vibrations of the brush are attenuated by the vibration-isolating plate and the vibration absorbing material.
However, in the first solution, there is a disadvantage of accelerated wearing of each brush, resulting in a reduced lifetime of the brush.
In the second solution, the vibration-isolating plate and the vibration absorbing material need to be used, disadvantageously increasing the number of components and the cost.
Furthermore, during rotation of the commutator 53 at a low rotational speed, the brushes 54 and the commutator 53 are placed under a high-friction state that is close to a static friction state. Thus, the self-excited vibrations of the brushes 54 are increased, resulting in generation of relatively large noises.
FIG. 16A shows levels of noises measured for the brush holder 52, and FIG. 16B shows frequency analysis for these noises. During these measurements, the commutator 53 is rotated at a relatively low rotational speed of 15 [rpm].
As indicated by an arrow B in FIG. 16A, it will be understood that the use of the previously proposed brush holders 52 causes relatively large levels of noises. Furthermore, as indicated by an arrow C in FIG. 16B, it will be understood that particularly large noises are observed in a range of 6 to 7 [kHz]. Thus, reduction of the noises at the above low rotational speed of the commutator 53 has been demanded.
The present invention addresses the above disadvantages. Thus, it is an objective of the present invention to provide a brush holding device that can reduce noises without changing a material of brushes and without increasing the number of components.
To address the above disadvantages, there is provided a brush holding device including a dielectric resin circuit board and at least one metal brush holder secured to the circuit board at its base portion. The circuit board is secured to a motor housing. Each brush holder includes a receiving recess that receives a brush and a spring for urging the brush against a commutator. One of each brush holder and the circuit board or the housing has at least one engaging portion integrally formed therein for achieving resilient engagement between each brush holder and the circuit board or the housing.